Agricultural Biotechnology Center, Gödöllő, Hungary.
PLoS One. 2012;7(2):e32497. doi: 10.1371/journal.pone.0032497. Epub 2012 Feb 23.
The Salmonella genomic island 1 (SGI1) is a 42.4 kb integrative mobilizable element containing several antibiotic resistance determinants embedded in a complex integron segment In104. The numerous SGI1 variants identified so far, differ mainly in this segment and the explanations of their emergence were mostly based on comparative structure analyses. Here we provide experimental studies on the stability, entrapment and variant formation of this peculiar gene cluster originally found in S. Typhimurium.
METHODOLOGY/PRINCIPAL FINDINGS: Segregation and conjugation tests and various molecular techniques were used to detect the emerging SGI1 variants in Salmonella populations of 17 Salmonella enterica serovar Typhimurium DT104 isolates from Hungary. The SGI1s in these isolates proved to be fully competent in excision, conjugal transfer by the IncA/C helper plasmid R55, and integration into the E. coli chromosome. A trap vector has been constructed and successfully applied to capture the island on a plasmid. Monitoring of segregation of SGI1 indicated high stability of the island. SGI1-free segregants did not accumulate during long-term propagation, but several SGI1 variants could be obtained. Most of them appeared to be identical to SGI1-B and SGI1-C, but two new variants caused by deletions via a short-homology-dependent recombination process have also been detected. We have also noticed that the presence of the conjugation helper plasmid increased the formation of these deletion variants considerably.
CONCLUSIONS/SIGNIFICANCE: Despite that excision of SGI1 from the chromosome was proven in SGI1(+)Salmonella populations, its complete loss could not be observed. On the other hand, we demonstrated that several variants, among them two newly identified ones, arose with detectable frequencies in these populations in a short timescale and their formation was promoted by the helper plasmid. This reflects that IncA/C helper plasmids are not only involved in the horizontal spreading of SGI1, but may also contribute to its evolution.
沙门氏菌基因组岛 1（SGI1）是一个 42.4kb 的整合可移动元件，包含几个抗生素抗性决定因素，嵌入在一个复杂的整合子片段 In104 中。迄今为止，已经鉴定出许多 SGI1 变体，它们主要在这个片段上有所不同，对它们出现的解释大多基于比较结构分析。在这里，我们提供了对最初在鼠伤寒沙门氏菌中发现的这个特殊基因簇的稳定性、捕获和变体形成的实验研究。
方法/主要发现：通过分离和共轭测试以及各种分子技术，检测了来自匈牙利的 17 株鼠伤寒沙门氏菌 DT104 分离株中沙门氏菌群体中新出现的 SGI1 变体。这些分离株中的 SGI1 被证明能够完全有效地进行切除、通过 IncA/C 辅助质粒 R55 进行共轭转移，并整合到大肠杆菌染色体中。构建了一个陷阱载体，并成功地应用于捕获质粒上的岛屿。对 SGI1 分离的监测表明该岛具有高度的稳定性。在长期繁殖过程中，没有积累 SGI1 缺失的分离株，但可以获得几种 SGI1 变体。它们中的大多数与 SGI1-B 和 SGI1-C 相同，但也检测到了两种由短同源依赖性重组过程引起的新变体。我们还注意到，辅助质粒的存在大大增加了这些缺失变体的形成。
结论/意义：尽管已经证明 SGI1 从染色体上的切除在 SGI1(+)沙门氏菌群体中是可能的，但仍无法观察到其完全缺失。另一方面，我们证明，在短时间内，在这些群体中，几个变体，包括两个新鉴定的变体，以可检测的频率出现，并且它们的形成是由辅助质粒促进的。这反映出 IncA/C 辅助质粒不仅参与了 SGI1 的水平传播，而且可能对其进化也有贡献。
